The present invention relates generally to the field of diffusion bonding of cracks and joints in metal substrates, and especially to diffusion bonding for nickel alloy- or cobalt alloy-containing substrates used in the manufacture of, for example, Industrial Gas Turbine, Aeroderivative Gas Turbine and Aircraft Gas Turbine components.
In a modern gas turbine engine, components such as blades, vanes, combustor cases and the like are usually made from nickel and cobalt alloys. Nickel and cobalt-based superalloys are most often used to fabricate gas turbine parts because of the high strength required for long periods of service at the high temperatures characteristic of turbine operation.
In use, these components sometimes experience cracking as a result of thermal fatigue, pitting from corrosion/oxidation processes, and dents from foreign object impacts. In order to prolong the useful life of the components, it is desirable to fill the cracks and other damage with a material that will bond to the substrate alloy, and will restore, at least to a substantial extent, both the shape and the strength of the cracked metal or component.
U.S. Pat. No. 3,496,630 (Duff et al.) describes a process for joining parts by brazing in which the joint is filled by a mixture of two metal powders having different melting points. The joint is heated to a temperature at which one of the powders melts to braze the joint. The two powders then merge by diffusion to form an alloy with a higher melting point.
U.S. Pat. No. 4,008,844 (Duvall et al.) describes a process in which cracks are filled with a mixture of metal powders, one of which contains boron as a melting point depressant. The repair is heated to a temperature at which that powder initially melts, then solidifies isothermally as the boron diffuses away. Duff et al.""s and Duvall et al.""s processes have been found in practice to be most suitable for repairing cracks of width 25 xcexcm to 250 xcexcm (0.001xe2x80x3 to 0.010xe2x80x3).
U.S. Pat. No. 5,156,321 (Liburdi) describes a process in which joints and cracks in nickel-based alloy components are filled with a nickel alloy powder of similar composition to the base alloy of the components. This powder is partially solid state sintered to form a solid but porous fill in the crack. Then, a layer of braze alloy of similar composition, but with the addition of at least 1% B and 1% Si, or up to 10% Si, to reduce its melting point is applied. This is then sintered at a temperature at which the braze alloy melts, forming a non-porous surface that seals the crack. The repair is then subjected to hot isostatic pressing, which collapses the porous interior of the repair, allowing it to diffusion-bond and form a solid repair. In some examples, the braze layer is applied before the first sintering step: the first sintering step is then at a temperature below the melting point of the braze.
Miglietti et al., Liquid Phase Diffusion Bond Repair of Westinghouse 501F, Row 3 Vanes, ASME paper 2000-GT-339, reports experimental results using a single-stage liquid-phase diffusion sintering process to repair cracks in a component made of X-45 cobalt superalloy. The composition of the filler paste used is not explained in detail, but it apparently contains Co, Ni, Cr, Al, Ta, W, Zr, Ti, and B. It has been found necessary with this process to maintain the component at liquid sintering temperatures for 24-48 hours to promote diffusion of the boron away from the filler. With this composition the boron is the primary melting point depressant.
The repairs reported by Miglietti et al. had tensile and yield strengths allegedly equivalent to the values quoted in the Metals Handbook for the base alloys being repaired, but had a low ductility, typically 25-51% of the values quoted in the Metals Handbook for the base alloy. This low ductility is believed to be due to the presence of boride phases. Ni3B has a Vickers hardness with a 50-g load of 883 HVN. At the time, this was not believed to be a problem. However, we have now discovered that increasing the ductility of the filling material in this sort of repair can substantially improve the performance of the repaired component in actual use. Low cycle fatigue (LCF) and thermal fatigue (TF) properties are strongly influenced by ductility, in that a material with higher ductility will have superior LCF and TF properties. Phosphorus and silicon have also been used as melting point depressants, but these also form hard, brittle eutectic phases. Phosphides and silicides are even more brittle than borides, and the larger phosphorus and silicon atoms diffuse away less readily than boron atoms.
The invention provides processes for liquid phase diffusion bonding of metal or alloy parts that use less boron or other melting point depressants so as to reduce the formation of brittle phases in the joint.
In one aspect, the invention provides a process for liquid phase diffusion bonding of a metal or alloy substrate, comprising filling a gap with a metal or alloy powder substantially free from melting point depressants. The powder may be similar in composition to the substrate, but is preferably MAR-M 247, MAR-M 247LC, or CM 247LC when repairing a Ni-based superalloy substrate, and MAR-M 509 when repairing a Co-based superalloy substrate. A braze consisting of a metal or alloy containing an effective quantity of a melting point depressant is applied over the alloy powder. In a first heating stage, a temperature above the liquidus of the braze and below the melting point of the powder is maintained for a period sufficient for the braze to infiltrate the spaces within the metal powder. In a subsequent heating stage a temperature below the liquidus of the fill is maintained while diffusion of the melting point depressant occurs.
In another aspect of the invention, a gap in a nickel- or cobalt-based metal or alloy substrate is filled with a powder substantially free from melting point depressants. The powder may be similar in composition to the substrate, but is preferably MAR-M 247, MAR-M 247LC, or CM 247LC when repairing a Ni-based superalloy substrate, and MAR-M 509 when repairing a Co-based superalloy substrate. A braze containing an effective quantity of a melting point depressant selected from the group consisting of zirconium and hafnium is applied over the metal or alloy powder. In a first heating stage a temperature above the liquidus of the braze and below the melting point of the powder is maintained for a period sufficient for the braze to infiltrate the spaces within the metal powder. In a subsequent heating stage, a temperature below the liquidus of the combined braze and powder is maintained while substantial diffusion of the melting point depressant occurs.
The effective quantity of the melting point depressant must reduce the melting point or liquidus of the braze sufficiently that the molten braze can infiltrate the spaces within the metal powder without the base metal of the component under repair being heated sufficiently to cause incipient melting or other deterioration. It will be appreciated by those skilled in the art that the exact amount of melting point depressant needed will depend not only on the specific compositions of the base metal, the filler powder, and the braze, but also on the intended use of the component under repair, which affects the amount of deterioration that can be tolerated.
Where the substrate is nickel-based, the braze preferably consists essentially of nickel with from 26% to 34% by weight of hafnium, corresponding to a melting point from 1190xc2x0 C. to 1260xc2x0 C., or from 11% to 19% or from 40% to 60% by weight of zirconium, corresponding to a melting point of from 1170xc2x0 C. to 1260xc2x0 C. A melting point of 1260xc2x0 C. is considered to be the maximum that is acceptable, to allow the liquid phase sintering to take place without incipient melting of the article being repaired. A braze containing both hafnium and zirconium, in quantities that are effective as a melting point depressant in combination, is also possible. A braze mixing Zr and/or Hf with other melting point depressants would be possible, but is less preferred, because using appreciable quantities of known melting point depressants such as boron will tend to introduce the undesirable properties that have been discovered to arise from those materials.
However, where this aspect of the invention is not applied, other aspects of the invention may be put into effect with, for example, a braze consisting of Ni, Co, Cr, Al, Ta, Mo, W, Y, Zr, Hf, and B, for a nickel-based substrate, or a braze consisting of Co, Ni, Cr, Al, Ta, W, Zr, Ti, and B, for a cobalt-based substrate. The braze may contain 1.5% B and 1.5% Zr and/or Hf. In that case, the B will be the primary melting point depressant. 1.5% by weight of boron reduces the melting point by about 160xc2x0 C. 1.5% Zr and/or Hf has very little effect on the melting point of the braze, reducing it by no more than 40xc2x0 C. The Zr is useful because it improves the elongation, the stress rupture life, and the overall life of the joint. The Hf is useful because it strengthens the xcex3xe2x80x2 phase (in which Hf is highly soluble), because it is an active carbide former and forms a uniform dispersion of carbides along the grain boundaries, because it increases the oxidation resistance of the brazed joint, and because it facilitates coating the brazed joint with an aluminide coating. Hf and Zr also form ductile Ni5Hf and Ni5Zr phases, which improve the overall ductility of the repair.
It is believed that other melting point depressants, including scandium, selenium, and titanium may be usable instead of or in addition to Zr and/or Hf. However, although published data indicate that those elements have a suitable effect on the melting point of Ni and/or Co, it is not yet certain which of them form alloy phases with the desired ductility and other mechanical properties. The ductility of the repaired joint should be at least 3%, and a ductility of at least 5% is very strongly preferred.
In another aspect of the invention, a gap in a nickel- or cobalt-based metal or alloy substrate is filled with a powder similar in composition to the substrate and substantially free from melting point depressants. A braze containing an effective quantity of a melting point depressant is applied over the metal or alloy powder. In a first heating stage a temperature above the liquidus of the braze and below the melting point of the powder is maintained for a period sufficient for the braze to infiltrate the spaces within the metal powder. In a subsequent heating stage, a temperature below the liquidus of the combined braze and powder is maintained while substantial diffusion of the melting point depressant occurs. The melting point depressant is selected from materials that form with the substrate material phases having a ductility at least comparable with the ductility of the substrate material.
In a further aspect of the invention, a method of liquid phase diffusion bonding of a metal or alloy substrate comprises filling a gap with a metal or alloy powder and an effective quantity of a melting point depressant selected from the group consisting of zirconium and hafnium. In a first heating stage a temperature above the liquidus of the braze and below the melting point of the metal or alloy powder is maintained for a period sufficient for the braze to infiltrate the spaces within the metal powder. In a subsequent heating stage a temperature below the liquidus of the braze is maintained while substantial diffusion of the melting point depressant occurs.
The temperature below the liquidus of the braze may be achieved either by reducing the process temperature, or by allowing the liquidus temperature to rise as the melting point depressant diffuses away and as the metal or alloy powder melts or diffuses into the braze and dilutes the melting point depressant.
In a further aspect of the invention, there is provided a braze for liquid phase diffusion bonding of a metal or alloy substrate, comprising: a melting point depressant selected from the group consisting of hafnium and zirconium; and a metal that forms a eutectic with the melting point suppressant; both in effective quantities.
It is an object of the invention to provide a process for liquid phase diffusion bonding that uses boron or other melting point depressants in smaller quantities than hitherto.
It is a further object of the invention to provide a process for liquid phase diffusion bonding that uses melting point depressants other than boron, silicon, or phosphorus that form ductile, rather than brittle, phases in the final joint.